The present invention relates to an electrophotosensitive material and, more particularly, to a digital electrophotosensitive material which is used in image forming apparatuses such as electrophotographic copying machine, facsimile and laser beam printer.
In image forming apparatuses such as electrophotographic copying machine, facsimile and laser beam printer, various organic photosensitive materials having the sensitivity in a wavelength range of a light source used in said apparatuses. Although a digital technique has recently been introduced into image forming apparatus, a red semiconductor laser (LD) and a light emitting diode (LED) are mainly used as the light source for digital image forming apparatus and light having a long wavelength of about 600 to 830 nm (orange light, red light and light in a near infrared range) are emitted from the light source and, therefore, it is strongly required to develop an organic photosensitive material which is excellent in sensitivity in these wavelength ranges.
Intense interest has been shown towards phthalocyanines (TiOPc) as an electric charge generating material having high sensitivity in a near infrared range. Particularly, a multi-layer type electrophotosensitive material using α type or Y type titanyl phthalocyanine (α-TiOPc, Y-TiOPc) or a mixed crystal of TiOPc and hydroxy metal phthalocyanine as an electric charge generating material has already been put into practical use.
On the other hand, a single-layer type electrophotosensitive material containing an electric charge generating material and an electric charge transferring material in a single photosensitive layer has the following advantages. That is, the single-layer type electrophotosensitive material is excellent in productivity because of its simple layer construction, as compared with a multi-layer type electrophotosensitive material comprising a conductive substrate and an electric charge generating layer and an electric charge transferring layer formed separately on the conductive substrate, and can inhibit the occurrence of layer defects during the formation of the photosensitive layer, and also the single-layer type electrophotosensitive material has improved optical characteristics because of less interface between layers and can be used as both of positive and negative charging type electrophotosensitive materials.
Therefore, there have been made various studies on the single-layer type electrophotosensitive material using the above-mentioned phthalocyanines as the electric charge generating material. However, there arises a problem that a single-layer type electrophotosensitive material having high sensitivity can not be obtained when using α-TiOPc, Y-TiOPc or a mixed crystal of TiOPc and hydroxymetal phthalocyanine as the electric charge generating material.
The reason is as follows. That is, a binder resin such as polycarbonate, polyarylate, polyester, polystyrene or polymethacrylate ester used in the formation of the photosensitive layer has low affinity with TiOPc or the mixed crystal and a dispersion medium of a coating solution for formation of a photosensitive layer is limited to a non-alcoholic solvent such as tetrahydrofuran, dioxane, dioxolane, toluene or methylene chloride taking account of the solubility of various materials constituting the photosensitive layer and, furthermore, the non-alcoholic solvent is a poor solvent to TiOPc or the mixed crystal.
Also there arises a problem that it becomes difficult to form a uniform photosensitive layer as a result of the occurrence of coagulative precipitation of TiOPc because of low dispersibility in the dispersion medium, and that the crystal form of TiOPc is transferred to a crystal form which is different from an expected crystal form after preparation of a dispersion because of low stability with a lapse of time in the dispersion medium.
Patent Documents 1 to 5 describe a coating solution prepared by incorporating TiOPc and specific azo pigments taking account of the dispersibility of a TiOPc-containing coating solution for formation of a photosensitive layer, and a single-layer type electrophotosensitive material (or photoconductor) using the same. Also Patent Documents 6 to 11 describe a single-layer type electrophotosensitive material comprising TiOPc and specific azo pigments.
However, the coating solution described in Patent Documents 1 to 5 still has a problem that the coating solution has poor storage stability. Furthermore, the electrophotosensitive material (photoconductor) described in Patent Documents 1 to 11 has a problem that it is inferior in charge stability and NOx resistance and such a problem drastically occurred under the high temperature atmosphere.
Patent Document 1: Japanese Published Unexamined Patent Application (Kokai Tokkyo Koho) No. 2000-47406 (see claims 1 and 2 and paragraph numbers [0013] to [0030])
Patent Document 2: Japanese Published Unexamined Patent Application (Kokai Tokkyo Koho) No. 2000-47407 (see claims 1 and 2 and paragraph numbers [0013] to [0029])
Patent Document 3: Japanese Published Unexamined Patent Application (Kokai Tokkyo Koho) No. 2000-147810 (see claims 1 and 2 and paragraph numbers [0021] to [0036])
Patent Document 4: Japanese Published Unexamined Patent Application (Kokai Tokkyo Koho) No. 2001-123087 (see claim 5 and paragraph numbers [0013] to [0026] and [0031])
Patent Document 5: Japanese Published Unexamined Patent Application (Kokai Tokkyo Koho) No. 2000-239553 (see claim 1 and paragraph numbers [0014] to [0027])
Patent Document 6: Japanese Published Unexamined Patent Application (Kokai Tokkyo Koho Hei) No. 7-175241 (see claim 1 and paragraph number [0004])
Patent Document 7: Japanese Published Unexamined Patent application (Kokai Tokkyo Koho Hei) No. 9-34148 (see claim 1 and paragraph number [0004])
Patent Document 8: Japanese Published Unexamined Patent application (Kokai Tokkyo Koho) No. 2000-147809 (see claim 2 and paragraph numbers [0020] to [0035])
Patent Document 9: Japanese Published Unexamined Patent Application (Kokai Tokkyo Koho) No. 2000-242011 (see claim 2 and paragraph numbers [0021] to [0040])
Patent Document 10: Japanese Published Unexamined Patent Application (Kokai Tokkyo Koho) No. 2002-55470 (see claims 1 and 2 and paragraph numbers [0022] to [0036])
Patent Document 11: Japanese Published Unexamined Patent Application (Kokai Tokkyo Koho Hei) No. 7-199493 (see claim 1 and paragraph numbers [0028] to [0029])
Thus, it is required to obtain an electrophotosensitive material, which has high sensitivity to a digital light source and also has high performances, by preparing a coating solution for formation of a photosensitive layer, which is excellent in dispersibility of phthalocyanines, stability in a dispersed state and stability with a lapse of time, and using the coating solution.
An object of the present invention is to provide an electrophotosensitive material which realizes uniform dispersion of phthalocyanines in a photosensitive layer and has high sensitivity to a digital light source, and also excellent in charge stability under the high temperature atmosphere, weatherability and NOx resistance, and preferably a single-layer type electrophotosensitive material.
Any azo pigments used in the inventions described in Patent Documents 1 to 11 are selected on the assumption that they exhibit the sensitivity in a broad visible range. Although a coupler residue including the naphthol structure portion is used in the azo pigments, the coagulability of the azo pigment itself is enhanced by the presence of an OH group having high polarity and also coagulative precipitation of the pigment occurs with a lapse of time in the coating solution for formation of a photosensitive layer because of low affinity with a binder resin such as polycarbonate.
Furthermore, it is considered that the OH group at the naphthol structure portion serves as an adsorption portion of an active gas such as NOx and, therefore, there arises a problem that an electrophotosensitive material containing the azo pigments is inferior in NOx resistance.
Since the azo pigments exhibit the sensitivity in a broad visible range, not only TiOPc incorporated as the electric charge generating material in the photosensitive layer, but also the azo pigments exhibit electric charge generating ability. However, since both of TiOPc and the azo pigments exhibit the electric charge generating ability, heat carrier is liable to occur and electric charge retention tends to be lowered. This leads to deterioration of the charge stability under the high temperature atmosphere.
Therefore, the present inventors have employed phthalocyanines having excellent sensitivity in a near infrared range during intensive study to achieve the above-mentioned object, and have studied intensively with a policy of incorporation of azo pigments in a single-layer or multi-layer type photosensitive layer for the purpose of improving the dispersibility of the coating solution for formation of a photosensitive layer.
As a result, they have found a new fact that there can be provided an electrophotosensitive material which realizes uniform dispersion of phthalocyanines in a photosensitive layer and has high sensitivity to a digital light source, and also excellent in charge stability under the high temperature atmosphere, weatherability and NOx resistance, and particularly a single-layer type electrophotosensitive material when using, as the azo pigments, (I) an insoluble azo pigment having no OH group such as hydroxyl group or carboxyl group wherein an absorbance in an absorption wavelength range of an electric charge generating material (phthalocyanine) is ⅓ or less of an absorbance in the wavelength of the electric charge generating material, or (II) an insoluble azo pigment having no OH group such as hydroxyl group or carboxyl group wherein an absorbance in a wavelength range of an exposure light source is ⅓ or less of an absorbance of the electric charge generating material (phthalocyanine) in the wavelength range in an image forming apparatus using the electrophotosensitive material of the present invention. Thus, the present invention has been completed.